This invention relates to a quick starting ultraviolet or UV emission unit that incorporates a novel electronic circuit to reduce the start-up phase to several hundred milliseconds. Additionally, the novel electronic circuit enables variations in output power from a minimum sustaining power to maximum output in discrete selectable steps. these features enable the UV unit to be particularly applicable to environments where initial start, shutown and restart of the UV emission are devised to be virtually instant.
Ultraviolet (UV) energy is widely used in many manufacturing processes, ranging from drying of inks, coatings and adhesives in paper, wood, metal, plastics, fiber optics, etc. to sterilization and disinfection of pharmaceutical products and treatment of waste waters.
The most common source of ultraviolet radiation is the electrode type mercury filled arc lamp. This is a quartz tube, filled with a inert gas (argon or xenon), and two electrodes, one at each end, which are connected to an appropriate power source. At room temperature the mercury enclosed in the lamp is in the liquid state. When an arc is applied to the electrodes, the enclosed inert gas is ionized and the lamp temperature rises causing evaporation of the mercury. Further electrical discharge through the mercury vapor produces a mercury plasma that discharges electromagnetic radiation in a wide band ranging from the lower UV to the INFRARED regions of the electromagnetic spectrum.
The time necessary to evaporate all the mercury within the lamp and to bring the mercury vapor to the correct operating pressure, is called the "warm-up" period. In the conventional lamp, this time can be as long as 5 minutes and in more modern systems it has been reduced to 45 seconds, is the state-of-the-art of today's commercial systems.
Once the electrode UV lamp is turned off, the impedance of the "hot" lamp is such that the lamp cannot be restarted until the mercury re-condenses, which requires cooling the lamp envelope toward room temperature, until the impedance is low enough for an arc to strike between the electrodes. The time required is called "restart time" and is typically of the order of 2 to 10 minutes.
The net result of these two characteristics (warm-up and restart), is that it is impractical to shut-off a electrode UV lamp during routine operation, every time the product stops underneath the lamp. Continuous impingement of the energy on a product that is not moving, will cause rapid increases of the temperature of the target, leading to distortion or burning of the product. This difficulty is overcome by the use of "shutters". Shutters mechanically block the energy of the lamp from the product when the process stops, and allow for immediate irradiation when production resumes.
UV systems with shutters pose serious limitations to decreasing the ultimate size of the irradiator. Size is important in many applications, but critical in printing press applications where the space between print stations is always limited. Furthermore, shutters introduced a mechanical complication which results in unreliability and added cost.
An electrodeless lamp, excited by microwaves to overcome the start-up/cool-down cycle of the electrode type lamp is disclosed in U.S. Pat. No. 3,911,318. The original lamp, which in practice was limited to a maximum of 10 inches of length, still had undesirable start-up characteristics. This type of lamp and its start-up time was improved by further enhancements as disclosed in U.S. Pat. No. 4,359,668. Even today, the start-up cycle is of the order of 2 to 4 sec with restart taking 10 seconds.
Other attempts at improving the start-up cycle of electrode type lamps, have been made and consist of adding a "third" wire to the outside of the lamp. As voltage is applied to the electrodes, high voltage pulses are applied to this wire, which is wrapped on the outside of the quartz tube, causing a faster ionization of the filler gas and thereby decreasing the start-up time. Commercial approach, and such systems are indistinctively referred to as "rapid start" dryers. Their actual starting times are of the order of 5 to 10 seconds.
A special electrical system, must be used to operate the electrode type mercury lamp. The lamp requires high voltage to initiate the arc and lower voltage to sustain it while it is operating. Since the mercury vapor tube has a voltage/current characteristic which is negative and non-linear, they require a constant wattage, ballast-type power supply. This power supply consists of a step-up transformer with a capacitance load. The lamp is connected in series to the secondary of the transformer. Once the lamp starts, the predominantly positive series impedance provides constant power and prevents the increasing current from destroying the lamp and the power supply. Once stabilized, the voltage across the lamp electrodes remains fairly constant and at a value lower that the open circuit voltage of the step-up transformer.
Once the lamp is in operation, it is convenient to be able to vary the power output, to match the speed of the commercial process to conserve energy and prevent excess heat built-up on the product. This is done by switching series capacitance in the power supply. Due to the high voltages and voltage transients involved in this switching process, mercury relays are conventionally used.
The quick starting ultraviolet emission unit of this invention substantially improves the operating characteristics in starting, restarting and regulating the output during operation. The unit accomplishes these desirable objectives inexpensively and in a manner allowing a compact size, often critical to installation in many environments.